Internal combustion engines, such as for example diesel engines, are often fitted with exhaust-gas turbochargers. For example, FIG. 1 shows a schematic illustration of an internal combustion engine 1 having an exhaust line 10 which is coupled to an exhaust-gas turbocharger 2. The exhaust-gas turbocharger has a turbine 4 which is driven by exhaust gas from exhaust line 10. The turbine 4 is coupled to a compressor 3 (together these components form turbocharger impeller unit) which compresses intake air from an intake air inlet 11. The compressed air discharged from the compressor 3 is fed to an intake line 9′ for the engine 1 in order to increase the air pressure in the engine 1, and thereby feed more air into the engine's cylinders when the cylinder's respective intake valves are open than would be fed into the cylinders if the engine is naturally aspirated. As a result of the turbocharger's supply of additional air into the engine cylinders, along with associated additional fuel from the engine's fuel injection system, the torque output of the engine is increased and the engine operates at a higher efficiency. Specifically, the additional pressure delivered by the turbocharger to the intake manifold results in greater pressure in the engine cylinder when the cylinder's intake valve closes. The greater mass of air present in the cylinder, when combined with additional fuel and ignited, results in higher combustion pressure, and thus higher piston force to be converted by the engine's crankshaft into higher engine torque output. In addition, the increased combustion mass and pressure generates a higher pressure and volume of exhaust gases, which in turn provides additional energy in the exhaust for driving the turbine of the turbocharger. The increased exhaust energy further increases the rotational speed of the turbocharger compressor and thereby further increases the amount of air being supplied to the cylinders to increase engine speed and torque output at an even more rapid rate. Those of ordinary skill in the art will recognize that although the foregoing and following discusses air for combustion arriving in the engine's cylinders via an intake manifold, the principles and concepts of the present invention are equally applicable to engines having alternative air supply volumes, such as engines in which the intake arrangements are such that each cylinder has an associated intake “chamber,” rather than receiving intake air from a common intake manifold.
A well known problem with the use of exhaust-gas turbochargers is that they cannot deliver a sufficient quantity of air in all operating states of the internal combustion engine, most notably in response to sudden acceleration demands at low engine rotational speeds. For example, in engines such as diesel engines having an exhaust-gas turbocharger, during a large acceleration demand the turbocharger typically cannot supply sufficient air flow to generate a desired amount of air pressure in the intake manifold due to the low engine speed and correspondingly low mass flow rate of air intake and exhaust output to drive the turbocharger. As a result, the internal combustion engine reacts slowly, with significant torque output and rotational speed increases occurring only after a notable delay after the accelerator pedal is pressed (an effect known as “turbo lag”).
Various solutions have been proposed to ameliorate the effects of “turbo lag,” including arrangements in which compressed air is supplied to the intake manifold of the engine. An example of such a “pneumatic booster” system is illustrated in FIG. 1. In this example, reservoir 13 stores compressed air generated by an air compressor 14. The compressed air is introduced into the intake line 9′ of the engine 1 in response to a demand for increase engine torque output during the transient period between the start of the acceleration demand and the time at which the turbocharger has built up enough pressure to equalize with the intake manifold pressure and begin to meet the torque output demand on its own.
The additional air supplied into the intake line 9′ from reservoir 13 has at least two primary effects. The additional combustion air fed to the cylinders of the engine 1 provides an immediate increase in engine torque output. The additional air also results in a more rapid increase in exhaust gas flow from the engine, which in turn helps the turbocharger turbine 4 to more rapidly increases its rotational speed, thus enabling the turbocharger compressor 3 to build pressure in the intake line 9′ faster. Further, the sooner the turbocharger compressor can supply enough pressure to support the torque output demand, the sooner the flow of additional air being supplied from reservoir 13 may be halted, preserving compressed air for other uses and reducing the duty cycle of the vehicle's air compressor.
The injection of compressed air from reservoir 13 in the FIG. 1 example takes place via an intake air control device 7. The intake air control device 7 is arranged between the intake line 9′ and either the compressor 3 of the turbocharger, or as shown in FIG. 1 the charge-air cooler 5 downstream from the compressor 3. The intake air control device 7, illustrated schematically in FIG. 2, is connected with an inlet 6 to the charge-air cooler 5 and with an outlet 9 to the intake line 9′.
A flap element 16 is located within the intake air control device 7, between the inlet 6 and the outlet 9. The flap element 16 can be adjusted by an adjusting motor 17 to close off the connection from the inlet 6 to the outlet 9 when compressed air is being injected into the intake line. Closing the flap prevents backflow of injected compressed air toward the turbocharger to help more quickly increase the pressure in the engine cylinders, which in turn increases the exhaust line pressure and resulting rate of turbocharger discharger pressure increase. In addition, closing the flap also provides a closed volume downstream of the turbocharger to further aid in building up the turbocharger discharge pressure.
A compressed air inlet 8 is connected to the outlet 9 to the reservoir 13 via a flow-regulating device 20. A controller 15 serves to control the flow-regulating device 20 and the adjusting motor 17. The control device 15 receives inputs from pressure sensors 18 and 19, which measure, respectively, an outlet pressure at the outlet 9 and an inlet pressure at the charge-air inlet 6.
In operation, the flow-regulating device 20 supplies compressed air to the engine intake manifold by opening the connection from the compressed-air inlet 8 to the outlet 9. At approximately the same time, the flap element 16 is closed to prevent flow of the injected compressed air from reservoir 13 back into the compressor 3 of the exhaust-gas turbocharger. As the injection of compressed air from reservoir 13 is ended, the flap element 16 is opened again to permit the now-sufficient compressed air supply from the discharge of turbocharger compressor 3 to flow into the intake line 9′.
While it has previously been known to inject compressed air into the intake manifold of an engine to reduce “turbo-lag,” work in this field has primarily concentrated on maximizing the amount of compressed air available to flow into the engine intake manifold, and on minimizing the response time from the initiation of the pneumatic boost event to the actual injection of compressed air so as to immediately begin to increase engine torque output and avoid undesired operator-perceived delays in delivery of torque from the engine.
One problem with prior pneumatic booster systems is the sometimes very abrupt increase in engine torque output at the beginning of a pneumatic boost event resulting from very rapid compressed air injection. Such sharp engine torque output transients may also be experienced at the subsequent termination of compressed air injection, and when the intake flap is switched open to resume turbocharger output to the engine. These transients can create significant discomfort to the vehicle operator and passengers.
Another problem with previous pneumatic booster systems is that, in the rush to quickly boost engine torque output until the turbocharger has built up sufficient pressure, regulatory limits such as pollution emissions limits may be exceeded. The sudden application of excessive pneumatic boost also has the potential to impose sudden loads on the engine components. For example, sudden application of excessive pneumatic boost can apply a large amount of torque to the vehicle drivetrain which may approach engine, transmission and/or drive axle stress limits. Excessive pneumatic boost may also generate a sudden high volume, high pressure flow of exhaust gases from the engine which can cause the speed of the turbocharger turbine-compressor assembly to rise to high levels. Similarly, sudden compressed air injection and accompanying increased exhaust gas flow can create the potential for over-pressuring the engine's intake air intercooler and its associated piping.
A further problem with previous pneumatic boost systems is the potential for over-injection of compressed air, and consequent depletion of the vehicle's compressed air reserves below the minimum amount needed to ensure operability of critical vehicle safety systems, such as air brakes, as well as other vehicle systems. One approach to minimizing this problem is to procure and install larger air compressors and compressed air storage vessels which are capable of meeting both the needs of critical air-consuming systems and the anticipated additional demands of the pneumatic booster injection system. However, this approach has its own problems, including increased cost and weight penalties for larger and more numerous air handling components, increased fuel consumption due to the increased vehicle weight and the need to consume more of the engine's power output to drive a larger compressor, and space constraints which inhibit the designer's ability to add additional reservoirs.